The invention relates to a filter element comprising a support element to which a flexible filter medium is applied. The invention further relates to a filter comprising a housing in which a filter element of the foregoing type is installed. The invention additionally relates to a method of backwashing a filter of the aforedescribed type.
Filter elements of the foregoing type are known. For instance, DE 198 33 381 A1 discloses a filter element comprising a helical spring as a support element to which a filter foil is mounted. This filter foil contains fine slots that allow the fluid to be filtered to flow through and retain the particles to be filtered out. As an alternative to this foil, screen netting may also be used for filtering.
These filters are used, for example, as backwash filters. Backwashing serves to clean the influent side of the filter medium when it becomes dirty and the openings through which the fluid passes become clogged. The filter medium on the support element is subject to mechanical loads due to the backwash pulses as well as resulting from any pulsating fluid flow. Over time, the alternating loads applied to the filter medium by the fluid to be filtered cause mechanical fatigue in the filter medium, which can lead to failure. The filter foil may tear, so that the purity of the filtrate is compromised.
It is the object of the present invention to provide a filter element comprising a support element and a filter medium mounted thereto, which is reliable in operation and has a long service life.
This object and other objects are attained in accordance with the present invention by providing a filter element comprising a cylindrical support element which is permeable to a fluid to be filtered, and a filter medium fixed to the support element which retains oversize particles contained in the fluid to be filtered, wherein the filter medium is fixed to the support element in such a way that relative movement between the filter medium and support element due to fluid flow through the filter element is prevented.
In accordance with a further aspect of the invention, the objects are achieved by providing a filter comprising an openable housing having an inlet for a fluid to be filtered and an outlet for filtered fluid, and a filter element disposed in the housing between the inlet and the outlet, the filter element comprising a cylindrical support element which is permeable to a fluid to be filtered, and a filter medium fixed to the support element which retains oversize particles contained in the fluid to be filtered, wherein the filter medium is fixed to the support element in such a way that relative movement between the filter medium and support element due to fluid flow through the filter element is prevented.
In accordance with another aspect of the invention, the objects are achieved by providing a method of backwashing a filter according to the invention comprising opening a discharge valve at a sludge outlet so that fluid from the filter flows in a reverse direction through the filter element and cleans the filter element, whereby sludge removed by backwashing from the filter element is discharged from the sludge outlet together with a portion of fluid, and thereafter closing the discharge valve so that fluid in the filter flows in a normal filtering direction, wherein the entire backwashing process takes less than 2 seconds per filter element.
The filter element according to the invention is characterized in that the filter medium is fixed to the support element, particularly a support tube, in such a way that any relative movement between filter foil and support element is prevented. This is true even if the fluid flow results in pulsating stress on the filter medium. It has been shown that mechanical loading of the filter medium is primarily produced by relative movements on the support element or by vibrations of parts of the filter medium excited by pulsations in the fluid to be filtered. A pulsating load can also be generated by backwashing with previously filtered fluid in order to clean the filter element. Critical loading of the filter medium is also caused by deformation due to indentations or buckling of unsupported partial areas of the filter medium. This may occur when the pressure difference on the filter medium is reversed, e.g., during backwashing of the corresponding filter. If the relative movement between filter medium and support element is prevented, however, the filter medium is simultaneously stabilized by the support element to the point where vibrations of the filter medium are also effectively prevented.
The flexible filter medium is disposed around the support element. To this end, the support element preferably is cylindrical to assure uniform bending of the filter medium. However, the cross section of the support element may also deviate from the circular form. Elliptical cross sections, for example, are also feasible. Screen mats made, for example, from woven wires may be used as the filter medium. Another option is to use slotted or perforated foils. These foils are extremely thin, and the slots or holes are suitable for allowing the fluid to be filtered pass through while the oversize particles in the fluid are retained. The size of the holes or slots directly influences the size of the particles being retained. Flow through the filter medium from the outside toward the inside is just as feasible as from the inside toward the outside. Backwashing is achieved by reversing the flow direction of the fluid.
In accordance with one specific embodiment of the invention, the filter medium contacts at least 3% of the support area provided by the support element without any play between the filter medium and the contacted areas of the support element. Contact occurs in different support regions, which are uniformly distributed over the entire support area. Tolerances between the support regions result, for example, from production inaccuracies of the support element. In the major part of the remaining support area, a radial clearance tolerance of a maximum of 0.2 mm is permitted with respect to the lateral surface formed by the support element. This assures that due to tolerances between support element and filter medium the support regions are randomly distributed over the support area and that the respective support regions are close enough to one another that any vibration of the filter medium between the individual support regions is effectively prevented.
The support area provided by the support element is smaller than the lateral surface area of the support element. The lateral surface area is provided with openings to allow the filtered fluid to pass from the clean side of the foil on the exterior of the support tube to the interior or vice versa. Thus the support area is formed by the segments between the openings. A support tube may, for example, comprise a helical spring. Another possibility is to use a perforated metal sheet that is bent into a tube. It is particularly advantageous to use so-called slot or gap tubes. They comprise a helically coiled strand having a, for example, triangular cross section. If desired, the support tube may be provided with axial supports to impart greater stability. The support area may advantageously be minimized to the edge of the triangular cross section of the strand so that the largest possible portion of the lateral surface is not covered by the support element, but is available directly for filtering.
It has been found that even if the permissible tolerance of 0.2 mm between filter medium and support element is exceeded over a maximum of 15% of the support area, a uniform distribution of the support regions on the support area is assured. The actual clearance tolerance may be exceeded, for instance, due to a fold in the filter foil created when the foil is bent around the support element. Such deformations, however, have an increased intrinsic rigidity due to their geometry, which also helps prevent vibrations in the filter medium. Consequently, an additional support by the support regions in these zones of the filter medium is not required.
In accordance with another specific embodiment of the invention, the filter medium is applied to the support element in such a way that only tensile forces act in the filter medium in tangential direction with respect to the circumference of the support element. This is achieved by mounting the filter medium on the support element under tensile stress. Tensile forces are then superimposed on any compression forces that may occur due to the curvature of the filter medium around the support element. These tensile forces must thus be greater than the largest compression force that occurs on the inside surface of the cylindrical filter medium due to the bending moment of the filter medium.
It is particularly advantageous if the tensile forces in the foil are large enough so that they also exceed the compression forces in the foil that occur as a result of the pulsating stress applied to the foil by the fluid. Any change in the pressure on the foil can cause the foil briefly to contract, which may produce compression forces. To prevent these forces from briefly detaching the foil from the support element, the tensile force present in the filter foil may simply be increased.
The tensile forces in the filter medium cause the filter medium to conform better to the support element. This makes it possible to reduce the radial clearance tolerance so that more support regions for the filter medium are created on the support area. Thus, the assembly between filter medium and support element in accordance with the invention becomes more stable against vibrations or any relative movement between the two components.
In accordance with one specific embodiment, the filter element is constructed in such a way that the tensile stress as viewed over the radial thickness of the filter medium has an average value of at least 5 N/mm2. The average value is calculated from the superposition of any bending moment that results from the elasticity of the foil bent around the supporting element and the tensile stress that is created in the filter medium by the assembly process. It has been found that an average value of at least 5 N/mm2 is sufficient to prevent any undue stress on the filter medium. The degree by which the filter medium has to be prestressed ultimately depends on the individual case, i.e., on the intensity of the pulsating load or the pressure changes on the filter element.
A particular embodiment of the filter element is obtained if the filter foils used as the filter medium are between 0.1-1 mm thick. In these foils, tensile stresses of at least 8 N/mm2 along the radially outward facing surface of the filter foil provide adequate protection of the filter foil against impermissibly high loads. This value can be readily measured in a quality control operation to assure production of reliable filter elements.
An advantageous embodiment of the invention results if the edges of the filter medium, which join it along the longitudinal dimension of the filter element to form a cylindrical body, are soldered together. This creates a strong joint of the filter medium and at the same time seals its edges. In addition, this soldered connection can permanently absorb any tensile stresses acting on the filter medium in tangential direction. This effect can be further enhanced by enlarging the surface of the soldered connection with the use of a reinforcement strip placed over the joint edges of the filter medium. This creates a gap between the reinforcement strip and the filter medium, which can be filled by the solder material.
Compared to standard overlapping soldering of the filter foil, the use of a reinforcement strip has the great advantage that no shoulder is created between the filter medium and the support element. In terms of the inventive concept, this further improves the fit of the filter medium on the support element. The filter medium is butted edge-to-edge and then soldered together with the reinforcement strip. The strip may be fixed radially either to the outside or the inside of the filter medium. If the strip is fixed to the inside, a corresponding groove must be formed in the filter medium to avoid a shoulder formed by the filter medium resting on the support element.
The filter element of the invention can be inserted into a filter having an openable housing. The filter element is inserted into the housing in such a way that an influent side of the filter, which is provided with an inlet, is separated from the effluent side of the filter, which is provided with an outlet. Advantageously, the filter housing can additionally be equipped with a backwash mechanism that reverses the flow direction through the filter medium. This permits simple cleaning of the influent side of the filter medium. The particles removed in the cleaning operation must be discharged from the filter housing.
In accordance with a further embodiment of the invention, fast valves with switching times of less than 0.4 seconds are provided at least as discharge valves for the sludge removed in the cleaning operation. These valves, which can for instance be configured as solenoid valves, permit pulsed backwashing of the filter foil. This increases the cleaning effectiveness. The use of fast valves for filter elements with filter foils is possible only if the foils are fixed to the support element as described above. Otherwise the quick load changes due to pulsed cleaning would cause the filter foil to fail as described above, as a result of the cyclical, high mechanical loads. The advantage of pulsed cleaning is the small amount of fluid required for the backwash process. The fluid for backwashing is already filtered, so that a greater backwash would create losses and would make the filtering process less effective. Reducing the backwash amount lowers the process costs for cleaning the fluid to be filtered.
The valves may be controlled by a controller, which makes it possible optimally to adapt the cleaning process to the respective conditions under which the filter is used. The backwash intervals as well as the characteristics of the backwash process are variable. In electrically actuated valves, the backwash process can for instance be controlled via the flow of the actuating current.
Normally, the backwash filters are operated as a series of several filter elements. During backwashing, only one of the filter elements is thus out of operation while the others continue to be available for filtering the fluid to be filtered. Due to the very short backwash pulses, however, the use of a single filter element is also feasible for certain applications. In these cases, pulse cleaning interrupts the filter process only very briefly, which in certain applications has no effect.
According to one specific embodiment of the invention, the intake valves mounted on the influent side of the filter element are also fast valves with short switching times. This permits a particularly effective configuration of the backwash process. During the backwash process the intake valves are closed, and the discharge valves are open. This causes a back flow from the clean side of the filter through the discharge valves, so that dirt collected on the influent side of the filter element is cleaned off and discharged through the sludge outlet. This avoids liquid losses at the intake of the filter device.
A particularly advantageous process for operating the filter according to the invention comprises opening the valves involved in the backwash process, discharging the usually sludge-like filtrate, and closing the valves. The entire process requires less than 2 seconds per filter element. This short duration of the process has the aforedescribed advantages of pulsed cleaning of the filter element. During this process, at least the discharge valves of the filter elements that are to be cleaned are opened. If intake valves sealing the influent side of the filter element with respect to the intake lines are provided in addition, these valves are also closed. A time shift in opening/closing the intake and discharge valves can produce particularly advantageous cleaning effects that must be determined as a function of the corresponding application.
The short periods involved in pulsed cleaning also increase the capacity of the filter system and make it possible to filter fluids with a high dirt concentration. In these cases, frequent cleaning is necessary. This can be provided, since the individual cleaning pulses are very short, so that the cleaning only marginally decreases the filter capacity of the individual filter elements over time.
These and other features of preferred embodiments of the invention are set forth in the claims as well as in the description and the drawings. The individual features may be implemented either alone or in combination in embodiments of the invention or in other fields of use and may represent advantageous embodiments that are protectable per se, for which protection is hereby claimed.